Visible-Light-Mediated Reactions under Mild Conditions

Abstract

Visible-light-mediated reactions have been incorporated into various organic synthesis schemes. Recently, owing to the important focus on environmental issues, the development of an environmentally friendly and mild synthetic methodology has become urgently needed. In this review, we summarize our recent approaches to developing the green mode of visible-light-mediated reactions that are available under mild reaction conditions without the use of toxic transition-metal-based catalysts or harsh reaction conditions. Relevant applications in the synthesis of valuable small molecules, including bioactive compounds, have also been highlighted.

Introduction

The use of visible light to effect chemical transformations has been a long-term challenge for chemists. As an example, in 1912, Giacomo Ciamician reported “The Photochemistry of the Future” in Science.¹ Afterward, Kellogg provided a basis for modern photoredox catalysis by reporting a reductive amination reaction in 1978.² Interestingly, however, not many studies were reported until the 2010s. Recently, visible-light-mediated photoredox catalysis has attracted a great deal of attention in organic synthesis.³ In these types of reactions, visible light is converted into chemical energy by engaging in single-electron transfer (SET) with substrates to generate reactive intermediates. Based on the unique reaction mode of visible-light-mediated transformations, a wide range of reactions have been developed over the past several years.

In recent years, green activation modes have been explored in this field owing to the growing interest in the development of environmentally benign synthetic methods. The most common and general approach to green photoredox catalysis is the use of organic photocatalysts. Various organophotocatalysts, including newly developed catalysts and traditional organic dyes, such as rose bengal, eosin Y, and 4CzIPN (2,4,5,6-tetrakis(9H-carbazol-9-yl)-isophthalonitrile) (Figure 1), have been employed in numerous reactions.^3b,4

Recently, many research groups have focused on the advantages of organic photocatalysts because their structures can be modified by a rational approach to realize unprecedented reactivity or improve the catalytic activity. As an alternative green approach, catalyst-free reactions have been developed. These latter types of reaction modes include the use of photoactive starting compounds or in situ excitation of the substrates under visible-light irradiation. The substrates thus activated form electron donor−acceptor (EDA) complexes, obviating the need for photoredox catalysts.⁵ This strategy relies on associating electron-donor with electron-acceptor substrates, such as Lewis bases and acids, respectively. The EDA complex can absorb visible light, undergoing an excitation process, which triggers a single-electron transfer (SET) that can generate radical intermediates. In addition to the above-mentioned approaches, green synthetic methods using natural substances such as molecular oxygen have also been studied. Our group has focused on the development of novel, visible-light-mediated reactions under mild reaction conditions. Herein, we summarize our approaches for realizing the green mode of visible-light-mediated reactions (Figure 2).

Conclusions and Outlook

The development of green synthetic methods is an important goal in the field of visible-light-mediated reactions. In this short review, we introduced our approaches for carrying out valuable organic transformations under mild reaction conditions such as visible-light-mediated aerobic oxidations using molecular oxygen. Additionally, we expanded the scope of singlet-oxygen-mediated reactions by developing oxidative C–S bond cleavage reactions. Moreover, using EDA complex-based synthetic strategies, we developed catalyst-free reactions through the intermediacy of binary or ternary EDA complexes. Various mechanistic studies have been conducted to understand the reaction mechanisms. However, further studies are required to directly analyze EDA complexes. In this review, applications of these approaches to the synthesis of valuable small molecules were also highlighted. Further studies are in progress in our laboratory to develop novel visible-light-mediated syntheses based on green approaches. Future directions would include the structural modification of organic photocatalysts and the development of novel visible-light-active starting compounds to expand the range of available reactions in this field.

Acknowledgment

Anna Lee thanks her co-workers for their contributions to the development of the reactions described in this article. This research was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean government (MIST) (2023R1A2C1005301 and 2022R1F1A1064110).